The removal of selenium from waste water is oftentimes a difficult challenge. Particularly difficult and complicated is the removal of selenium from process water in the mining, coal gasification and petroleum refining industries. For example, sour crude oils and coals produced or mined from formations containing seleniferous marine shales often contain significant amounts of selenium. During the refining of such crude oil, the refinery process water is found to contain relatively large amounts of selenocyanate ions derived from sodium or ammonium selenocyanate which is formed during the refinery process. Coal gasification process waters may also contain selenium as selenocyanate ion. Where cyanide leaching is utilized in the mining industry, selenium can be complexed with cyanide as selenocyanate.
The present invention relates to a process for treating waster water (e.g. refinery process water) containing selenocyanate ions to remove and/or significantly reduce the amount of selenocyanate ions for example to less than about 0.1 mg/L.
The present invention accomplishes the foregoing by adding to waste water containing selenocyanate ions, a water soluble cupric salt, the waste water also containing at least a stoichiometric amount, based on the cupric salt, of a reducing agent for the cupric salt. The cupric salt can be a neutral or basic salt but the acid salt is preferred. The cupric ion of the cupric salt (which is water soluble) is reduced by the reducing agent to the cuprous ion which in turn reacts with the selenocyanate ion to produce cuprous selenocyanate in which the Se is immobilized in the cyanate complex. This complex is water insoluble and forms a precipitate which is easily removed from the waste water.
It was very surprising that the formation of the cuprous ion insitu would be effective in removing and/or reducing the amount of selenocyanate ion from waste water since the direct addition of a cupric salt is not effective in appreciably reducing the selenocyanate ion concentration in waste water. This is due, at least in part, to the fact that cupric selenocyanate is relatively water soluble.
For a number of years the problem of removing selenium from waste water has been a concern. It is therefore not surprising that a number of methods have been disclosed in the prior art for the removal of various forms of selenium. For example, U.S. Pat. No. 5,089,141 discloses the removal of "organoselenium" from water by adding a composition containing nickel and/or copper and magnesium and/or aluminum. The patent states that the addition of this composition forms elemental selenium which is a water insoluble precipitate and may be removed from the waste water.
U.S. Pat. No. 4,405,464 discloses a process for the removal of selenium ions from mine water prior to discharge of the aqueous stream into the ground water system. According to the patent, "it is known" that selenium can be removed from aqueous compositions if the selenium is present in the selenite state by, for example, the addition of an iron salt such as ferric or ferrous sulfate, chloride or hydroxide. The '464 patent discloses using metallic iron powder to remove selenite by reducing it. The reduced selenite then forms a precipitate and is removed from the aqueous composition.
U.S. Pat. No. 4,806,264 discloses a method of removing selenium ions from an aqueous solution by adding to the solution ferrous ions to reduce the selenium ions to elemental selenium.
U.S. Pat. No. 3,933,635 discloses removal of selenium from acidic waste water using zinc.
U.S. Pat. No. 5,510,040 is directed to the removal of selenium compounds from aqueous liquids by using polydithiocarbamate compounds.
U.S. Pat. No. 4,935,146 discloses, among other things, removing selenium from waste water with the compound having the formula shown in column 4, line 4 of the patent A cupric salt does not fall within the formula disclosed in this patent.
U.S. Pat. No. 4,790,945 discloses a removal of hydrogen selenide from a liquid, a heavy metal non-selenide salt which is stated to precipitate the hydrogen selenide as a heavy metal selenide salt. The patent goes on to state that, conveniently, the heavy metal is copper, because copper selenide has very low solubility and that cupric chloride is preferred.